Lecture 10, Ch. 23

Lecture #10

Date ________

• Chapter 23~ The Evolution of Populations

Population genetics •

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Population: a localized group of individuals belonging to the same species Species: a group of populations whose individuals have the potential to interbreed and produce fertile offspring Gene pool: the total aggregate of genes in a population at any one time Population genetics: the study of genetic changes in populations Modern synthesis/neo-Darwinism “Individuals are selected, but populations evolve.”

Hardy-Weinberg Theorem •

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Serves as a model for the genetic structure of a nonevolving population (equilibrium) 5 conditions: 1- Very large population size; 2- No migration; 3- No net mutations; 4- Random mating; 5- No natural selection

Hardy-Weinberg Equation • p=frequency of one allele (A); q=frequency of the other allele (a); p+q=1.0 (p=1-q & q=1-p) • P2=frequency of AA genotype; 2pq=frequency of Aa plus aA genotype; q2=frequency of aa genotype;

p2 + 2pq + q2 = 1.0

Microevolution, I • A change in the gene pool of a population over a succession of generations • 1- Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)

Microevolution, II • The Bottleneck Effect:

type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

Microevolution, III • Founder Effect: a cause of genetic drift attributable to colonization by a limited number of individuals from a parent population

Microevolution, IV • 2- Gene Flow:

genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

Microevolution, V • 3- Mutations:

a change in an organism’s DNA (gametes; many generations); original source of genetic variation (raw material for natural selection)

Microevolution, VI • 4- Nonrandom mating: inbreeding and assortive mating (both shift frequencies of different genotypes)

Microevolution, VII • 5- Natural Selection: differential success in reproduction; only form of microevolution that adapts a population to its environment

Population variation • Polymorphism:

coexistence of 2 or more distinct forms of individuals (morphs) within the same population

• Geographical variation: differences in genetic structure between populations (cline)

Variation preservation • •

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Prevention of natural selection’s reduction of variation Diploidy 2nd set of chromosomes hides variation in the heterozygote Balanced polymorphism 1- heterozygote advantage (hybrid vigor; i.e., malaria/sicklecell anemia); 2- frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)

Natural selection • Fitness: contribution an individual makes to the gene pool of the next generation • 3 types: • A. Directional • B. Diversifying • C. Stabilizing

Sexual selection • Sexual dimorphism:

secondary sex characteristic distinction

• Sexual selection:

selection towards secondary sex characteristics that leads to sexual dimorphism